The present invention relates to a light emitting diode (LED) having a double hetero-structure and more particularly to an LED that has a high optical power and can be used at a large current.
A highly efficient LED having a double hetero-structure as shown in FIG. 21 is known. FIG. 21 is a vertical sectional view showing an AlGaInP LED in which layers are lattice-matched with a GaAs substrate 1. The structure of each layer in the LED is as follows:    Substrate 1:            made of an n-type GaAs            Buffer layer 2:            made of n-type GaAs            N-type cladding layer 3            made of n-type (Ga0.3Al0.7)0.5In0.5P                    impurity: Si, impurity concentration: 1×1018 cm−3, and            thickness: 1 μm                            Light-emitting layer 4:            made of p-type (Ga0.7Al0.3)0.5In0.5P                    thickness: 0.5 μm                            P-type cladding layer 5:            made of p-type Al0.5In0.5P                    impurity: Zn, impurity concentration: 5×1017 cm−3, and            thickness: 1 μm                            First current diffusion layer 7:            made of p-type Ga0.3Al0.7As                    impurity: Zn, impurity concentration: 1×1018 cm−3, and            thickness: 1 μm                            Second current diffusion layer 8:            made of p-type Ga0.3Al0.7As                    impurity: Zn, impurity concentration: 3×1018 cm−3, and            thickness: 6 μm                            Contact layer 9:            made of p-type GaAs            An n-side electrode 10 is formed on the underside of the n-type GaAs substrate 1. A p-side electrode 11 is formed on the p-type GaAs contact layer 9.
The n-type GaAs buffer layer 2 is intended to eliminate defects of the n-type GaAs substrate 1 and influence of contaminants in the substrate and is not required if the n-type GaAs substrate 1 is surface-treated favorably. The p-type GaAs contact layer 9 has a Gads structure not containing Al to facilitate an ohmic contact between the p-type GaAs contact layer 9 and the p-side electrode 11. The GaAs composing the contact layer 9 does not transmit light generated from the p-type (Ga0.7Al0.3)0.5In0.5P light-emitting layer 4, but no problem is raised because the contact layer is formed immediately below the p-side electrode 11.
Sharp K. K. has recently proposed an LED, a vertical sectional view of which is shown in FIG. 22, to achieve higher reliability than the above LED (Japanese Patent Application No. 10-338656). The structure of each layer in the LED is as follows:    Substrate 21:            made of n-type Gas            Buffer layer 22:            made of n-type GaAs            N-type cladding layer 23:            made of n-type (Ga0.3Al0.7)0.5In0.5P                    impurity: Si, impurity concentration: 1×1018 cm−3, and            thickness: 1 μm                            Light-emitting layer 24:            made of p-type (Ga0.7Al0.3)0.5In0.5P                    thickness: 0.5 μm                            First p-type cladding layer 26:            p-type (Ga0.5Al0.5)0.5In0.5P                    impurity: Zn, impurity concentration: 1×1017 cm−3, and thickness: 0.2 μm                            Second p-type cladding layer 27:            made of p-type Al0.5In0.5P                    impurity: Zn, impurity concentration: 5×1017 cm−3, and thickness: 1.0 μm                            Current diffusion layer 28:            made of p-type Ga0.9In0.1P                    impurity: Zn, impurity concentration: 1×1018 cm−3, and thickness: 7 μm                            Contact layer 29:            made of p-type GaAs        
An n-side electrode 30 is formed on the underside of the n-type GaAs substrate 21. A p-side electrode 31 is formed on the p-type GaAs contact layer 29.
A p-type cladding layer 25 is formed as a two-layer structure consisting of the p-type (Ga0.5Al0.5)0.5In0.5P first cladding layer 26 and the p-type Al0.5In0.5P second cladding layer 27. Accordingly, it is possible to prevent a p-type impurity from diffusing to the p-type (Ga0.7Al0.3)0.5In0.5P light-emitting layer 24 although the p-type impurity has a large impurity gradient and is liable to diffuse when electric current flows through the LED for a long time. Thus it is possible to prevent deterioration of the optical power of the LED.
The LED is used in the form of a chip. Conventionally, an LED wafer is divided into chips of a size of 200 μm-300 μm by 200 μm-300 μm. In the above LEDs, the p-type GaAs contact layers 9, 29 and the p-side electrodes 11, 31 are formed circular and disposed at the center of each chip. FIG. 23 shows a planar configuration of the chip.
The above LEDs have the following problem: Electric current flows immediately below the p-side electrodes 11, 31. Both the p-side electrodes 11, 31 and the p-type GaAs contact layers 9, 29 disposed under the p-side electrodes 11, 31 are opaque. Thus, the p-side electrodes 11, 31 and the contact layers 9, 29 intercept light coming from parts of the p-type (Ga0.7Al0.3)0.5In0.5P light-emitting layers coming from parts of the p-type (Ga0.7Al0.3)0.5P light-emitting layers 4, 24 that are located immediately below the p-side electrodes 11, 31. Thus, the light coming from those parts cannot be taken out to the outside. Accordingly, the above LEDs have a low light-emitting efficiency.
The LED chips are conventionally used at electric current LED having an intensity of several milliamperes to 50 mA. If the LED chip is used for an electric current having intensity higher than that, the optical power of the LED chip will saturate and characteristics will deteriorate with the passage of a current.